The invention relates to a method and a device in a homodyne receiver to be used in radio, tele, and data communication systems such as portable cellular phones, cordless phones, pagers, carrier frequency systems, TV cable systems, etc. Receivers in this technical field should preferably be small, lightweight and inexpensive.
The first generation of cellular systems relied on analogue frequency modulation for speech transmission, and several standards have been developed, such as NMT450, NMT900, AMPS, and ETACS.
The second generation of cellular systems follows three different standards: in Europe and some countries in Asia and Australia--Global System For Mobile Communications (GSM), in north America--American Digital Cellular (ADC), and in Japan--Pacific Digital Cellular (PDC). These systems all employ digital voice transmission and some digital services such as facsimile transmission and short messages.
To make the portables smaller and less expensive much research has been done to increase the level of integration of different parts in the phone.
Prior art receivers that have been used in this technical field were of the conventional heterodyne type. For applications in small low cost mobile communication systems these receivers suffer from high production costs caused by expensive and non-integrable RF and IF components such as band pass filters. To overcome such drawbacks alternative receivers have been developed. These receivers are based on the direct conversion principle. The local oscillator frequency is equal to the received carrier frequency and, consequently, the received signal is converted to the base band in one single step. This concept was first introduced for SSB-receivers but can be used in many different types of modulation, particularly for digital quadrature modulation schemes.
In a homodyne receiver or a zero-IF-receiver the received signal and the local oscillator operate at exactly the same frequency. Since there are no intermediate frequencies (IF) many filters can be omitted or simplified. The operation of the homodyne receiver can be described as follows. The RF signal of center frequency f.sub.c and bandwith BW.sub.rf is amplified with a low noise amplifier to improve the total noise figure of the receiver. The signal is then split and down converted to DC by mixers in both channels. The down converted spectrum is folded over itself and spans from DC to 1/2 BW.sub.rf. The low frequency signals I and Q provided by the mixers are then filtered to remove any adjacent channel and amplified to set the noise floor. The I and Q signals or the quadrature signals will allow basically any type of modulation when an appropriate signal processing is utilized.
A major drawback of direct conversion receivers is spurious emission. The main source of spurious emission in a direct conversion receiver is local oscillator leakage. In an ordinary super heterodyne receiver the local oscillator leakage to the antenna is attenuated by the first receiver bandpass filter. In a direct conversion receiver this is not the case since the local oscillator frequency lies within the passband of this bandpass filter. At least two types of leakage are present in a direct conversion receiver. The first type is wire bound leakage, and a second type is radiated leakage caused by parasitic coupling between leads and/or bonding wires.
Different methods have been suggested to overcome the problems with spurious emissions. W092/01337 discloses a direct conversion receiver comprising an antenna, a RF-filter, an amplifier and a mixer. A local oscillator, operating at a subharmonic of the received frequency, provides a signal to the mixer. A standard type mixer is used and a normal drawback of such a mixer, is that harmonics will be generated in the mixer when a signal of a frequency lower than the received signal is fed to the mixer from the LO is utilized to obtain the wanted signal in the mixer. Even though the local oscillator operates at a subharmonic of the received signal also harmonics will be generated. Some of these harmonics will in fact correspond to the received signal, and spurious emission will occur at some level.
In DE 3240565 another type of homodyne receiver is disclosed. The LO of this receiver is a controllable oscillator that generates a signal with a frequency forming a multiple of the receiver frequency. The generated signal is then phase shifted 180.degree. and divided to the frequency of the RF-signal. A major drawback in a receiver having a LO operating at a multiple of the received frequency is the difficulties to obtain the required characteristics of the LO. For instance the power consumption of such an oscillator will be difficult to match to the demands of a low total power consumption of the receiver.